PNNL

The Science

Simulating quantum chemical systems using quantum approaches will be fundamental to efficiently leveraging emerging quantum computing technologies but will require developing effective theoretical frameworks suited to the specific nuances of quantum systems. Researchers developed new approaches for simulating the quantum behavior underlying X-ray spectra and open quantum systems, thus enabling more predictive simulations of the quantum dynamics of chemical systems. These approaches are broadening the type of simulations that can be done on future quantum computers while maintaining computational efficiency.

The Impact

Quantum systems are complex, with interacting electrons, environmental effects, and emergent phenomena that can be difficult to reproduce using classical theoretical approaches. To fully realize the potential of quantum computing for quantum simulation, researchers are developing theoretical approaches that focus on the distinct properties of quantum systems. These specialized frameworks can enable more accurate simulations with similar or lower computational cost when compared to standard approaches. 

Summary

Solving quantum problems requires effective quantum computing tools. A range of chemical systems exhibit complex and important quantum dynamics that are necessary to accurately model to gain a full picture of the system. Researchers applied a quantum focus to better model X-ray spectroscopy data and open quantum systems.

X-ray spectroscopies can reveal quasiparticle energies and many-body satellite features. Interpreting the spectra is difficult because core-hole creation activates quantum phenomena that standard treatments can miss. Researchers developed a new time-dependent double coupled-cluster (TD-dCC) approach that combines ground-state and ionized-state correlations while retaining efficient real-time Green's-function simulations. The approach uses cost-effective Baker-Campbell-Hausdorff approximations and component analysis to identify the hole-mediated excitation pathways responsible for satellite formation. Benchmarks on molecular systems such as H₂O and CH₄ show that TD-dCC more accurately reproduces exact quasiparticle weights and satellite structures. The results also outline a quantum signal-processing route for computing core-hole Green's functions on future quantum computers.

Open quantum systems often interact with structured environments, producing memory effects that ordinary Markovian Lindblad dynamics cannot capture. Researchers developed a fractional-calculus framework that embeds power-law memory directly into quantum master equations while preserving complete positivity and trace. Through Bochner-Phillips subordination, the fractional dynamics are represented as averages over standard Lindblad evolutions evaluated at random operational times, creating a unified bridge from Liouville dynamics to Lindblad dynamics to memory-kernel models. The framework connects fractional resolvents to Nakajima-Zwanzig kernels, hierarchical equations of motion self-energies, and influence-functional descriptions, providing a compact surrogate for long-memory environments. Benchmarks on the spin-boson model show that the fractional parameters can capture non-exponential decoherence and algebraic relaxation across structured paths. This approach offers a non-phenomenological, interpretable, and computationally efficient route for simulating memory, decoherence, and dissipation on classical and future quantum hardware.

Contact

Bo Peng, Pacific Northwest National Laboratory, peng398@pnnl.gov 

Funding

This work was supported by the Early Career Research Program of the U.S. Department of Energy (DOE), Office of Science, under Grant No. FWP 83466.

Support also was provided by a DOE, Office of Science, Early Career Research Program Award in the Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Computational and Theoretical Chemistry program under FWP Grant No. 83466. 

Pacific Northwest National Laboratory is operated by Battelle for DOE under Contract No. DE-AC05-76RL01830. Y.Z. acknowledges the support from the Laboratory Directed Research and Development program of Los Alamos National Laboratory (LANL). LANL is operated by Triad National Security, LLC, for the National Nuclear Security Administration of the DOE (Contract No. 89233218CNA000001).